Tuning the Nonradiative Electron-Hole Recombination with Defects in Monolayer Black Phosphorus.

We use nonadiabatic (NA) molecular dynamics to demonstrate that the nonradiative electron-hole recombination is delayed and accelerated by the Stone-Wales (SWs) and phosphorus divacancy (DV-(5|7)) defects in monolayer black phosphorus (BP). Both types of defects increase the bandgap by 0.1 eV without creating midgap states. Driven by P-P stretching vibrations, the recombination proceeds within 1 ns in the SW and within 100 ps in the DV-(5|7), respectively, which occurs within 332 ps in BP. The SW defect slows down recombination because the notably reduced NA coupling combined with a large bandgap competes to the long-lived coherence. In contrast, the DV defect accelerates recombination since long-lived coherence is superior to the slightly decreased NA coupling correlated with a tiny increased bandgap. The diverse time scales rationalize the broad range of charge carrier lifetimes reported experimentally. The study provides a strategy to engineer excited-state dynamics for improving the BP-based optoelectronics.